Image reading apparatuses such as scanners, digital still cameras and video cameras have become popular, as the Internet has become prevalent. The image reading apparatuses have different mechanisms and physical properties from image output apparatuses such as displays and printers. Therefore, the image data obtained from an image reading apparatus generally requires correction such as gamma correction to present a picture with fidelity.
Provided that X denotes input pixel data and Y denotes output pixel data, the Gamma correction can be expressed in the form Y=XY, or other empirical curve. The function representation is hard to realize by hardware, so a look-up table is often used to enhance processing speed. The size of the gamma correction table depends on the resolution (bit number) of the input pixel data and output pixel data. The gamma correction table requires 4K bytes of storage space for 12-bit input data and 8-bit output data. The gamma correction table requires 64K bytes of storage space for 16-bit input data and 8-bit output data, which is not feasible for an ordinary platform.
The applicability of the look-up table is also limited by data accessing speed. Page mode accessing is not useful due to the randomness of pixel data. The data accessing time is 120 ns for external 60 ns DRAM.